1. Field of the Invention
The present invention relates to a photoactive compound having a novel structure and a photosensitive resin composition comprising the same. More specifically, the present invention relates to a photoactive compound that has a high UV absorption rate, is excellent in sensitivity and thermal processing characteristics, and is highly compatible with other components in a photosensitive resin composition, and a photosensitive resin composition comprising the photoactive compound.
2. Description of the Related Art
Photoactive compounds refer to materials that are decomposed to generate chemically active atoms or molecules when they absorb light. Photoactive compounds are widely used in photosensitive resin compositions due to their ability to generate chemically active species. Examples of such chemically active species include acids, bases and radicals. Particularly, a photoactive compound capable of generating radicals is used together with a radically polymerizable acrylic group for the purpose of improving the strength of a coating film.
A typical photosensitive resin composition can be used for the formation of a pattern by the following procedure. First, the photosensitive resin composition is applied to a substrate to form a coating. A particular portion of the coating is exposed to light through a photomask. The coating is developed to remove the unexposed portion, leaving a pattern. Photosensitive resin compositions are currently used in the production of photocurable inks, photosensitive printing plates, a variety of photoresists, including color filter photoresists for LCDs, photoresists for resin black matrices and transparent photoresists, etc. because they can be polymerized and cured upon being irradiated with light.
Photosensitive resin compositions have been used in traditional applications, such as notebook computers and mobile devices. In recent years, an increasing demand for high quality LCDs in diverse applications has extended the applicability of photosensitive resin compositions to the fabrication of liquid crystal display devices for televisions and monitors. There also exists a strong need for photosensitive resin compositions that rapidly respond to light and have excellent mechanical properties to improve the productivity and durability of liquid crystal display devices.
High reactivity to light, i.e. photosensitivity, of a photosensitive resin composition is a very important factor in forming a pattern by photolithography or an insulating protective film by full exposure. Further, column spacers acting as supports or an overcoat and a passivation film acting as protective films should have excellent mechanical properties in order to exert their inherent performance characteristics without damage to a liquid crystal display device by an external impact.
Under such circumstances, there is a need for photopolymerization initiators with high photosensitivity. Even a small amount of a highly photosensitive photopolymerization initiator in a photosensitive resin composition can sufficiently increase the sensitivity of the photosensitive resin composition, which reduces the generation of sources contaminating liquid crystal, increases the residual film ratio of a pattern and broadens the choice of other raw materials for the preparation of the composition.
Acetophenone derivatives, benzophenone derivatives, biimidazole derivatives, acylphosphine oxide derivatives, triazine derivatives and oxime ester derivatives are known as photopolymerization initiators suitable for use in photosensitive resin compositions. Of these, oxime ester derivatives become colorless when they absorb UV light, have the ability to generate radicals, and are very stable in compositions.
Oxime ester structure-based photoinitiators have been developed, for example, α-oxooxime derivatives, combinations of thioxanthone and oxime ester compounds, oxime esters using p-dialkylaminobenzene as a synergist, photoinitiators using β-aminooxime, and oxime ether photoinitiators containing one or more ethylenically unsaturated groups in the molecular structure. These early oxime derivatives have the problem of low photoinitiation efficiency and are inefficient in UV absorption when they have good color characteristics.
Numerous efforts have been made to improve the photoinitiation efficiency of oxime derivatives. However, the oxime derivatives fail to sufficiently shorten the processing time to a satisfactory level. Particularly, the oxime derivatives do not sufficiently satisfy the degree of cure of films having a high pigment concentration or a coating having a thickness as large as 2.5 μm. Accordingly, there remains a difficulty in forming fine patterns using the oxime derivatives. Further, patterns formed using the oxime derivatives do not meet the critical dimension (CD) and mechanical strength required in final products.
Irgacure OXE 01 and Irgacure OXE 02, oxime ester-based photopolymerization initiators commercially available from Ciba Specialty Chemical, that are currently in use have markedly improved sensitivity. However, the Irgacure series are economically unfavorable because the sensitivity is not sufficient considering their high price. Another problem of Irgacure OXE 01 and Irgacure OXE 02 is poor storage stability.
Thus, there is a need to develop a photoactive compound as a photoinitiator that can efficiently absorb UV light even when used in a small amount and is excellent in sensitivity, thermal processing characteristics and solubility, and a photosensitive resin composition comprising such a photoactive compound.